Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, at least one transmitter such as a current electrode (e.g., a current source or sink) is used in conjunction with a diffuse return electrode (such as the tool body or mandrel of the logging tool). Conventionally, the electrode injecting current has been known as a “button.” A measured electric current flows in a circuit that connects a current source to the transmitter, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, a current flow is induced within the earth formation, such as, for example, by using an antenna within the measuring instrument. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the first category.
Electrical logging devices may be operable in one or more modes. In one mode, a constant current is maintained at a measuring electrode while a voltage (i.e., electric potential difference between said electrode and the tool's return) is measured; in another mode, the voltage of the measuring electrode is held constant and the current is measured. Ideally, if current is varied to maintain a constant voltage, the resultant current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, if current is maintained constant through the circuit, ideally speaking the voltage of the measurement electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current. These electrical measurements may be correlated with electrical properties of the borehole, such as resistivity parameters (e.g., a resistivity image of a volume of interest of the formation). Characterizing resistivity parameters of the formation facilitates optimization of further operations conducted in the formation, such as evaluation, drilling, or other exploration or completion operations of a typical oil or gas well.
Of course, the foregoing is a simplified and idealized description of physical relationships that are more complex in practical application. Those of ordinary skill in the art will appreciate that there are usually other variables to consider, for example the electrical characteristics of the instrumentation itself and the electrical characteristics of the environment under investigation. The prior art suggests innumerable approaches for addressing such non-idealized factors.
The borehole may be filled with a variety of downhole fluids including, for example, hydrocarbons and drilling fluids (“muds”). The composition of the drilling fluid may vary in accordance with formation operations being conducted, with example considerations including operating parameters, environmental conditions, and objectives associated with the operations. Drilling fluids may be generally characterized as water-based or oil-based.
Typically, water-based mud is electrically conductive, while oil-based mud is substantially non-conductive. It is known that oil-based drilling fluids may be used when drilling through water-soluble formations, and an increasing number of present day exploration prospects are believed to lie beneath water-soluble salt layers. The use of oil-based muds, which have a low electrical conductivity relative to water-based muds, makes the use of electrodes to inject electrical currents into a wellbore challenging.